Near-threshold photodissociation of cool OH to O + H and O + H.

We have measured the spectrum of laser photodissociation of OH molecular ions to O + H and O + H fragments for photon energies of 38 100-40 900 cm. The OH ions were stored as a fast beam (5.50 MeV) in the storage ring TSR for several seconds to achieve rovibrational cooling into the lowest rotations N'' = 0-11 of the vibrational ground state XΣ(v'' = 0), close to room temperature (≈300 K). The many resonances in the spectra reveal the energies, widths, and O/O branching ratios of 44 predissociating quasibound levels (Feshbach resonances) that lie between the fine-structure states of the O fragment and belong to the last, near-threshold vibrational states v' = 9 and 10 of the AΠ electronic state. For the AΠ substates, isolated levels with v' = 11 are observed and attributed to double-well distortions of these curves due to nonadiabatic interactions. Another five isolated levels are assigned to the v' = 0 and 1 states of the shallow 1Σ electronic state, borrowing oscillator strength from nearby AΠ levels. Together, the near-threshold levels deliver a new value D = 40 253.8(1.1) cm for the dissociation energy of OH. Through a two-step photodissociation process, 72 levels from the lower bound states AΠ(v' = 7-8) appear as well and are rotationally analyzed. The level energies are used to construct improved AΠ and 1Σ Born-Oppenheimer potentials. The totality of the spectral data (energies, widths, intensities, and branching ratios) can provide tight constraints for the potentials and nonadiabatic interactions assumed in future coupled-channel calculations of OH photodissociation or of the related charge-exchange reaction O + H → O + H.